/Engine/Plugins/Animation/LiveLink/Source/LiveLink/Public/LiveLinkClient.h
Whether or not a subject from the specific source is the enabled subject. Only 1 subject with the same name can be enabled. At the start of the frame, a snapshot of the enabled subjects will be made. That snapshot dictate which subject will be used for the duration of that frame.
virtual bool IsSubjectEnabled ( const [FLiveLinkSubjectKey](API\Runtime\LiveLinkInterface\FLiveLinkSubjectKey) & SubjectKey, bool bForThisFrame ) const